Electronic circuits, such as gas discharge lamp ballasts, generally contain a means for converting AC line voltage to DC line voltage. A conventional way of obtaining the DC voltage from the AC line voltage is shown, for example, in FIG. 1 of Capewell U.S. Pat. No. 4,222,096 wherein a bridge rectifier and a filter capacitor are employed. However, it is known that in such a circuit the power factor is relatively low (i.e., 40 to 70%). Referring to FIG. 6a of Capewell, the power factor of a rectifying network will be maximized when the phase difference between the line voltage and the line current is minimal and further when the duty cycle of the line current is maximized. The duty cycle of the line current is defined as the per unit time that current flows from the line during each half cycle of line voltage. The rms current increases for a lower duty cycle because the waveform of instantaneous current i(t) is more peaked or has a shorter conduction period. Therefore, the input current necessary to support input power is very high and can exceed the ratings of conductors and circuit breakers.
The lighting industry has long recognized the advantage of high power factor circuits, and thus a power factor greater than 90% has become an actual requirement of ballasts for gas discharge lamps. Conventional, or electromagnetic, ballasts for gas discharge lamps employ bulky transformers and inductors as ballasting and power factor correcting elements to achieve a power factor of about 80-90%; yet, these bulky magnetic components dissipate a lot of power, lowering the efficiency of the ballasts. Acting as an inductive load, an electromagnetic ballast generates high third harmonic current as well (usually more than 30%).
In contrast to conventional electromagnetic ballasts, electronic ballasts represent non-linear resistive loads to the AC line due to large electrolytic filter capacitors across the output of the rectifying elements. Consequently, electronic ballasts draw non-sinusoidal current from sinusoidal voltage sources. Thus, a third order damped low pass filter, so called power factor and low total harmonic distortion correction, is needed to correct the waveshape of the AC line current charging the electrolytics of electronic ballasts.
In addition to a high power factor circuit, industry standards require lighting circuits to have a low total harmonic distortion (THD). THD is defined in the following equation: EQU THD=SQRT(N.sub.2.sup.2 +N.sub.3.sup.2 +. . . )/N.sub.1
where, EQU SQRT=Square-root;
N.sub.n =the magnitude of the nth harmonic frequency;
and
N.sub.1 =the magnitude of the fundamental frequency.
In a simple DC power supply consisting of a bridge rectifier and a filter capacitor, the THD is high due to distortion in the current waveform which includes sharp peaks and conduction angle of less than 180 degrees. While present standards require a THD less than 32%, efforts are underway to require the THD limit to be less than 20%.
Capewell U.S. Pat. No. 4,222,096 proposes a circuit with a high power factor which minimizes the phase difference between the line voltage and the line current and which maximizes the duty cycle of the line current. The circuit illustrated in FIG. 5 of U.S. Pat. No. 4,222,096, for example, includes an inductor and a capacitor connected in series with an AC source and the AC side of a rectifying element. The inductance-capacitance circuit resonates at a frequency of about three to six times the frequency of the input power supply. While this circuit produces a DC voltage with a high power factor, the THD is relatively high due to a distorted input current waveform resulting primarily from a high third harmonic component.
Lesea U.S. Pat. No. 4,672,522 describes a power factor correcting network connected in series with the input of a DC power supply across the AC power line. The network constitutes a third harmonic trap and comprises a parallel combination of an inductor and a capacitor. The inductor and capacitor resonate at a frequency greater than three and less than four times the power line frequency. While the above network is considerably effective at power factor correction and third harmonic control, in practice, it has been found that the series-connected capacitor tends to resonate with the inductance of the power line conductor creating a ringing phenomena which increases the magnitude of higher order harmonics. Moreover, since the currents through the inductor and capacitor in such a network are 180 degrees out of phase, they tend to cancel out if both are equal in magnitude. This cancellation phenomenon occurs at the beginning of every half cycle resulting in a zero current angle THETA and less than 180 degree conduction angle which contributes to higher order distortion.